The ability to ensure a consistently high quality of manufactured semiconductor components, for example semiconductor wafers and dies, is increasingly crucial in the semiconductor industry. Semiconductor wafer fabrication techniques have been consistently improved to incorporate an increasing number of features into a smaller surface area of the semiconductor wafer. Accordingly, the photolithographic processes used for semiconductor wafer fabrication has become more sophisticated to allow the incorporation of increasing features to the smaller surface area of the semiconductor wafer (i.e. higher performance of the semiconductor wafer). Consequently, sizes of potential defects on semiconductor wafers are typically in the micron to submicron range.
It is evident that manufacturers of semiconductor wafers have an increasingly pressing need to improve semiconductor wafer quality control and inspection procedures to ensure a consistently high quality of manufactured semiconductor wafers. Semiconductor wafers are typically inspected for detecting defects thereon, such as presence of surface particulates, imperfections, undulations and other irregularities. Such defects could affect eventual performance of the semiconductor wafer. Therefore, it is critical to eliminate or extract defective semiconductor wafers during the manufacture thereof.
There have been advances in semiconductor inspection systems and processes. For example, higher resolution imaging systems, faster computers, and enhanced precision mechanical handling systems have been commissioned. In addition, semiconductor wafer inspection systems, methods and techniques have historically utilized at least one of brightfield illumination, darkfield illumination and spatial filtering techniques.
With brightfield imaging, small particles on the semiconductor wafer scatter light away from a collecting aperture of an image capture device, thereby resulting in a reduction of returned energy to the image capture device. When the particle is small in comparison with the optical point spread function of a lens or digitalizing pixel, brightfield energy from the immediate areas surrounding the particle generally contribute a large amount of energy relative to the particle, thereby making the particle difficult to detect. In addition, the very small reduction in energy due to the small particle size is often masked by reflectivity variations from the immediate areas around the particle thereby resulting in increased occurrences of false defect detection. To overcome the above phenomena, semiconductor inspection systems have been equipped with high-end cameras with larger resolutions, which capture images of smaller surface areas of the semiconductor wafer. However, brightfield images generally have a better pixel contrast and this is advantageous for estimating size of defects and when inspecting dark defects.
Darkfield illumination and its advantages are generally well-known in the art. Darkfield imaging has been employed with several existing semiconductor wafer inspection systems. Darkfield imaging typically depends on the angle at which light rays are incident on the object to be inspected. At a low angle to a horizontal plane of the object to be inspected (for example 3 to 30 degrees), darkfield imaging typically produces a dark image except at locations where defects, such as surface particulates, imperfections and other irregularities exist. A particular use of darkfield imaging is to light up defects which sizes are smaller than the resolving power of lens used to produce a brightfield image. At a higher angle to the horizontal plane (for example 30 to 85 degrees), darkfield imaging typically produces better contrast images compared to brightfield images. A particular use of such high angle darkfield imaging enhances contrast of surface irregularities on a mirror finish or transparent object. In addition, high angle darkfield imaging enhances imaging of tilted objects.
Light reflectivity of the semiconductor wafer typically has a significant effect on quality of image obtained with each of brightfield and darkfield imaging. Both micro and macro structures present on the semiconductor wafer affect the light reflectivity of the semiconductor wafer. Generally, amount of light reflected by the semiconductor wafer is a function of the direction or angle of incident light, the viewing direction and the light reflectivity of the surface of the semiconductor wafer. The light reflectivity is in turn dependent on wavelength of the incident light and material composition of the semiconductor wafer.
It is generally difficult to control the light reflectivity of semiconductor wafers presented for inspection. This is because the semiconductor wafer may consist of several layers of material. Each layer of material may transmit different wavelengths of light differently, for example at different speeds. In addition, layers may have different light permeabilities, or even reflectivity. Accordingly, it will be apparent to a person skilled in the art that the use of light or illumination of a single wavelength or a narrow band of wavelengths typically adversely affects quality of captured images. Need for frequent modification of the single wavelength or narrow band of wavelengths requires use of multiple spatial filters or wavelength tuners, which can be generally inconvenient. To alleviate such problems, it is important to use a broadband illumination (i.e. illumination of a wide range of wavelengths), for example broadband illumination of a range of wavelengths between 300 nm and 1000 nm.
Currently available wafer inspection systems or equipments typically use one of the following methods for achieving or capturing multiple responses during wafer inspection:
(1) Multiple Image Capture Devices with Multiple Illuminations (MICD)
The MICD uses a plurality of image capture devices and a plurality of illuminations. The MICD is based on the principle of segmenting the wavelength spectrum into narrow bands, and allocating each segmented wavelength spectrum to individual illuminations. During design of systems employing the MICD method, each image capture device is paired with a corresponding illumination (i.e. illumination source), together with corresponding optical accessories such as a spatial filter or a specially coated beam splitter. For example, wavelength of the brightfield illumination is limited between 400 to 600 nm using mercury arc lamp and spatial filter and the darkfield illumination is limited between 650 to 700 nm using lasers. The MICD method experiences disadvantages, for example inferior image quality and design inflexibility. Inferior image quality is due to varying surface reflectivities of inspected wafers, combined with the use of illuminations with narrow wavelengths. Design inflexibility occurs because the modification of the wavelength of a single illumination typically requires reconfiguration of the entire optical setup of the wafer inspection system. In addition, the MICD method typically does not allow capture of illuminations with varying wavelengths by a single image capture device without compromising the quality of captured images.
(2) Single Image Capture Device with Multiple Illuminations (SICD)
The SICD method uses a single image capture device for capturing multiple illuminations, either with segmented wavelengths or broadband wavelengths. However, it is not possible to obtain multiple illumination responses simultaneously while the wafer is in motion. In other words, the SICD method only allows one illumination response when the wafer is in motion. To achieve multiple illumination responses, the SICD method requires image captures while the wafer is stationary, which affects throughput of the wafer inspection system.
Semiconductor wafer inspection systems employing simultaneous, independent, on-the-fly image capture using broadband brightfield and darkfield or in general multiple illuminations and using multiple image capture devices are not presently available due to a relative lack of understanding as to actual implementation and operating advantages thereof. Existing semiconductor wafer inspection systems are employing either MICD or SICD as explained earlier. Equipments employing MICD do not use broadband illumination and suffer from inferior image quality and inflexible system setup. On the other hand equipments using SICD experience diminished system throughput and are typically incapable of obtaining on-the-fly simultaneous multiple illumination responses.
An exemplary existing semiconductor wafer optical inspection system that utilizes both brightfield illumination and darkfield illumination is disclosed in U.S. Pat. No. 5,822,055 (KLA1). An embodiment of the optical inspection system disclosed in KLA1 utilizes MICD as explained earlier. It uses multiple cameras to capture separate brightfield and darkfield images of semiconductor wafers. Captured brightfield and darkfield images are then processed separately or together for detecting defects on the semiconductor wafer. In addition, the optical inspection system of KLA1 captures brightfield and darkfield images simultaneously using separate sources of brightfield and darkfield illumination. KLA1 achieves simultaneous image capture using segmentation of illumination wavelength spectrum, narrow band illumination sources and spatial filters for enabling capture of the brightfield and darkfield images. In the KLA1-optical system, one of the cameras is configured to receive darkfield imaging using narrow band laser and spatial filter. The other camera is configured to receive rest of the wavelength spectrum using brightfield illumination and a beam splitter with special coating. Disadvantages of the optical inspection system disclosed by KLA1 include unsuitability thereof for imaging different semiconductor wafers comprising a large variation of surface reflectivities due to segmentation of the wavelength spectrum. The cameras are tightly coupled with respective illumination and there is no flexibility of combining of more than one available illumination to enhance certain wafer types. One such type is having carbon coated layer on its front side and they exhibit poor reflection characteristics at certain illumination angle, for example using brightfield alone. It requires combination of brightfield and high angle darkfield illumination to view certain defects. Accordingly, the optical inspection system of KLA1 requires a plurality of light or illumination sources and filters for performing multiple inspection passes (multiple scan which in turn affects the throughput of the system) to thereby capture multiple brightfield and darkfield images.
Additional exemplary exiting optical inspection systems utilizing both brightfield and darkfield imaging are disclosed in U.S. Pat. No. 6,826,298 (AUGTECH1) and U.S. Pat. No. 6,937,753 (AUGTECH2). Darkfield imaging of the optical inspection systems of AUGTECH1 and AUGTECH2 utilizes a plurality of lasers for low-angle darkfield imaging, and a fiber optic ring light for high-angle darkfield imaging. In addition, the optical inspection system of AUGTECH1 and AUGTECH2 uses a single camera sensor and belongs to SICD method explained earlier. Accordingly, inspection of semiconductor wafers in AUGTECH1 and AUGTECH2 is performed either by brightfield imaging or by darkfield imaging or via a combination of both brightfield imaging and darkfield imaging wherein each of the brightfield imaging and darkfield imaging is performed when the other is completed. The inspection system of AUGTECH1 and AUGTECH2 is not capable of simultaneous, on-the-fly or while wafer is in motion and independent brightfield and darkfield imaging. Accordingly, multiple passes of each semiconductor wafer is required for completing inspection thereof, resulting in lowered manufacturing throughput and undue increase in utilization of resources.
In addition, several existing optical inspection systems utilize a golden image or a reference image for comparison with newly acquired images of semiconductor wafers. Derivation of the reference image typically requires capturing several images of known or manually selected “good” semiconductor wafers and then applying a statistical formula or technique to thereby derive the reference image. A disadvantage with the above derivation is presence of inaccuracies or inconsistencies in manual selection of the “good” semiconductor wafers. Optical inspection systems using such reference images typically suffer from false rejects of semiconductor wafers due to inaccurate or inconsistent reference images. With increasingly complex circuit geometry of semiconductor wafers, reliance on manual selection of “good” semiconductor wafers for deriving reference images is increasingly incompatible with increasingly high quality standards set by the semiconductor inspection industry.
Deriving a golden reference image involves many statistical techniques and calculations. Most of the statistical techniques are very general and have their own merits. State of the art of the currently available equipments uses either average or mean together with standard deviation to calculate a golden reference pixel. This method works well with known good pixels; otherwise any defect or noise pixel would interfere and affect a final average or mean value of the reference pixel. Another method is to use median and it has reduced interference due to noise pixel but is not possible to eliminate the effect of noise substantially. All of the available equipments try to reduce the error by applying different kinds of statistical techniques such as mean, median among others, but they do not have any special or user friendly sequence to eliminate the error. Such special sequence certainly helps to eliminate pixels which would affect the final reference pixel value.
U.S. Pat. No. 6,324,298 (AUGTECH3) discloses a training method for creating a golden reference or reference image for use in semiconductor wafer inspection. The method disclosed in AUGTECH3 requires “Known Good Quality” or “Defect Free” wafers. Selection of such wafers is manually or user performed. Statistical formulas or techniques are then applied for deriving the reference image. As such, accurate and consistent selection of “good quality” wafers is crucial for accurate and consistent quality of semiconductor inspection. Further, AUGTECH3 uses mean and standard deviation to calculate individual pixels of the reference image and presence of any defective pixel will lead to inaccurate reference pixel. The defective pixel occurs due to foreign matter or other defects, which would confuse the statistical calculation and lead to incorrect reference pixel. It will be apparent to a person skilled in the art that the method of AUGTECH3 is open to inaccuracies, inconsistencies and errors in inspection of the semiconductor wafers.
In addition, optical inspection system disclosed in AUGTECH3 uses a flash or strobe lamp for illuminating the semiconductor wafers. It will be appreciated by a person skilled in the art that inconsistencies between different flashes or strobes may occur due to numerous factors including, but not limited to, temperature differentials, electronic inconsistencies and differential flash or strobe intensities. Such differentials and inconsistencies are inherent even with “good” semiconductor wafers. Presence of such differentials would affect the quality of golden reference image if the system had not taken care of such differentials due to flash lamp. In addition, illumination intensity and uniformity varies across the surface of the semiconductor wafer due to factors including, but not limited to planarity of the wafer, mounting and light reflectivity at different positions of the surface. Without taking into account the variations in the flash intensity and the strobing characteristics of the lamp, any reference images generated in the above-described manner may be unreliable and inaccurate when used for comparing with captured images of different positions of the semiconductor wafers.
Variations in product specifications, for example semiconductor wafer size, complexity, surface reflectivity and criteria for quality inspection, are common in the semiconductor industry. Accordingly, semiconductor wafer inspection systems and methods need to be capable of inspecting such variations in product specifications. However, existing semiconductor wafer inspection systems and methods are generally incapable of satisfactorily inspecting such variations in product specifications, especially given the increasing quality standards set by the semiconductor industry.
For example, a typical existing semiconductor wafer inspection system uses a conventional optical assembly comprising components, for example cameras, illuminators, filters, polarizers, mirrors and lens, which have fixed spatial positions. Introduction or removal of components of the optical assembly generally requires rearrangement and redesign of the entire optical assembly. Accordingly, such semiconductor wafer inspection systems have inflexible designs or configurations, and require a relatively long lead-time for modification thereof. In addition, distance between objective lens of the convention optical assembly and semiconductor wafer presented for inspection is typically too short to allow ease of introduction of fiber optics illumination with differing angles for darkfield illumination.
There are numerous other existing semiconductor wafer inspection systems and methods. However, because of current lack of technical expertise and operational know-how, existing semiconductor wafer inspection systems cannot employ simultaneous brightfield and darkfield imaging for an inspection while the wafer is in motion, while still maintaining flexibility in design. There is also a need for semiconductor wafer inspection systems and methods for enabling resource-efficient, flexible, accurate and fast inspection of semiconductor wafers. This is especially given the increasing complexity of electrical circuitry of semiconductor wafers and the increasing quality standards of the semiconductor industry.